Thiocarboxylate-functional silanes are coupling agents which are used extensively in rubber applications such as tires and tire components. Several batch processes of making thiocarboxylate-functional silane are known. The batch processes include the reactions of alkali metal salts of mercapto-functional silanes with carboxylic acid chlorides or the reactions of alkali metal salts of alkylthioic acids with halo alkyl-containing silanes. The later process, the reactions of alkali metal salts of alkylthioic acids with halo alkyl-containing silanes, are advantageously carried out in a two-phase batch process in which an aqueous phase of the alkali metal salts of alkylthioic acids is reacted with a non-aqueous phase of the halo alkyl-containing silane, often in the presence of phase transfer catalyst. The two-phase batch process eliminates the need to handle solid alkali halide salts, which are a byproduct of the reaction, because these salts are soluble in the aqueous phase.
However, the two-phase batch process has the disadvantages of having to conduct washing of the non-aqueous phase, requiring long reaction times and forming meta-stable rag layers. The washing is needed to remove the phase transfer catalyst from the non-aqueous phase containing the reaction product. The phase transfer catalyst is an undesirable impurity in the product which may affect the curing of the rubber compositions in which the thiocarboxylate-functional silanes are used. The long reaction times results from the diffusion control transport of the alkali metal salts into the non-aqueous phase, and the need to carry out the reaction to completion before the next batch can be started. The meta-stable rag layer, which is an emulsified mixture of water, the reaction products and the phase transfer catalyst, make the separation of the two-phases difficult, often resulting in loss of reaction product.
The use of the two-phase batch process results in the process having the problems of being expensive, long and of limited capacity. Thus, there still exists a need for a process of making thiocarboxylate-functional silane that provides for efficient reaction of a two-phase reaction mixture, reduction of rag layers and very low amounts and advantageously, absence of phase transfer catalysts in the final product.